(1) Field of the Invention
The present invention relates to a switching power supply apparatus and a semiconductor device each of which regulates output voltages against input voltages by implementing switching using a switching element.
(2) Description of the Related Art
Switching power supply apparatuses have been widely used as power supply units of appliances for common household use, such as home electrical products, for purposes such as improvement of power efficiency by reducing power consumption. A switching power supply apparatus includes a semiconductor device which regulates (stabilizes) output voltages through switching operation using semiconductors (switching elements such as transistors).
Particularly, in recent years, there is a strong demand for switching power supply apparatuses which consume less power in a standby state, because reduction of power consumption of equipment such as home electric appliances in the standby state is attracting attention from the viewpoint of prevention of global warming.
Generally, in a low-load state such as a standby load, energy loss in switching power supply units is dominantly caused by switching loss due to switching operation. One of well known techniques for improving power efficiency under a light load such as a standby load is operating a power supply under intermittent-oscillation control.
FIG. 1 shows an exemplary configuration of a switching power supply device 900 including a semiconductor device with a conventional intermittent-oscillation control circuit. Operation during intermittent oscillation will be briefly described below with reference to a timing chart shown in FIG. 2. It is assumed that current-mode PWM control is performed during normal operation at, for example, a switching frequency of 100 kHz.
In the switching power supply device 900 configured as shown in FIG. 1, an output voltage Vout increases as an output current Iout decreases from a current in a rated-load state when the switching power supply device 900 is in a changing-load state shown in FIG. 2. An output voltage detection circuit 5 provides an FB terminal with a feedback signal which indicates the magnitude of the output voltage Vout (for example, an outflow current IFB which increases as the output voltage Vout increases). Based on the current IFB, the feedback control circuit 11 outputs a control voltage VEAO which indicates a smaller limiting value on a current flowing through a switching element 2 for a greater output voltage Vout.
When under a lighter load, an intermittent-oscillation control circuit 16 operates to provide a turn-on control circuit 18 with an enable signal which instructs to perform intermittent control of switching operation, so that intermittent-oscillation control starts under which switching operation of the switching element 2 is repeatedly executed and suspended as in a first standby state shown in FIG. 2.
When an output load current is smaller than in the first standby state, the switching operation changes to a second standby state in which the switching operation has a longer suspension period than in the first standby state. In other words, the lower the load is, the longer an intermittence control period, which includes an execution period and a suspension period of the switching operation of the switching element 2, is controlled to be. Such a control of intermittent oscillation performed under a light load improves power efficiency under the light load.
In such operation, even when the switching element 2 executes switching at a frequency of 100 kHz during normal operation, the frequency under intermittence control in an intermittent-oscillation control state which includes the execution period and the suspension period of the switching decreases to within an audible frequency range of 20 kHz or lower. This may cause a transformer or a capacitor, which is generally used in the switching power supply device, to generate audio noises.
In other words, intermittent-oscillation control under a light load is effective in improving power efficiency under the light load but has disadvantage of causing elements such as a transformer to generate noises when a frequency under intermittence control falls within an audible frequency range.
Well-known techniques for reducing such noises of the transformer include decreasing of a peak current of a switching element during intermittent oscillation, and bonding or impregnating of the transformers. However, decreasing of a peak current of a switching element during intermittent oscillation increases the number of switching cycles during the intermittent oscillation, resulting in worse power efficiency. The technique used with the transformer results in an increase in cost. Thus, reducing audible noises of a transformer while improving power efficiency under a light load may require much time and cost.
Patent reference 1 (Japanese Patent No. 4039362) discloses a conventional switching power supply device to address to such need for reduction of noises of a transformer under intermittent-oscillation control when under such a light load.
According to Patent Reference 1, a soft-start capacitor for start-up is repeatedly charged and discharged in order to perform intermittent oscillation, and a switching element is controlled so that it executes switching when an error-detecting signal being outputted instructs to change from a suspension state to an execution state of oscillation under intermittent-oscillation control, and so that it suspends switching when the error-detecting signal being outputted instructs to change from an execution state to a suspension state of oscillation under the intermittent-oscillation control in the case where the voltage across the capacitor is at a high level during the charge and the discharge are performed. In other words, the maximum frequency under intermittent-oscillation control is determined according to a frequency of cycles of charge and discharge of the soft-start capacitor for start-up.
With this control, a minimum intermittence period during intermittent oscillation is determined, and noises of the transformer are reduced.
Patent Reference 2 (Japanese Unexamined Patent Application Publication No. 2008-92793) discloses a switching power supply device which reduces noises of a transformer under intermittent-oscillation control when under a light load.
According to Patent Reference 2, a maximum intermittent control frequency during intermittent oscillation is determined by a switching frequency of a switching element and the number of switching cycles while oscillation is executed in the intermittent oscillation.
In one specific example, a switching frequency during normal operation is set to 30 kHz and there are at least four switching cycles while switching is executed in intermittent oscillation. The maximum intermittent control frequency is determined by the number of switching cycles included in an intermittence period, five switching cycles in this case: four consecutive switching cycles and one switching cycle suspended while switching is executed during intermittent oscillation. Thus, the maximum intermittent control frequency is 6 kHz. The maximum intermittent control frequency is thus determined, and noises of the transformer under intermittent-oscillation control are reduced.
In the switching power supply apparatuses disclosed in Patent Reference 1 and Patent Reference 2, noises of transformers under intermittent-oscillation control are reduced by determining a minimum intermittence period in intermittent oscillation, in other words, determining a maximum intermittent control frequency in intermittent oscillation.
For the conventional switching power supply apparatus disclosed in Patent Reference 1, however, it is difficult to optimize both soft-starting time and a maximum intermittent control frequency for reducing noises of transformers during intermittent oscillation because the intermittent oscillation frequency is determined using the soft-start capacitor for start-up. For example, if the soft-start capacitor with a large capacitance is provided in order to prolong the soft-start time, the suspension period during the intermittent oscillation is prolonged, thus increasing output ripple during intermittent-oscillation control.
For the conventional switching power supply apparatus disclosed in Patent Reference 2, a minimum period for intermittence control is determined by controlling the number of switching cycles within a switching execution period during intermittent oscillation based on a switching frequency of the switching element. In other words, a maximum intermittent control frequency is a frequency obtained by dividing the switching frequency. This configuration necessitates a device, such as a counter which counts the number of switching cycles or pulses of a clock signal having a fixed frequency so as to specify the minimum period for intermittence control. Furthermore, a degree of freedom in specifying a maximum frequency for intermittence control is small because a maximum intermittent control frequency is determined by dividing the frequency of the clock signal.
It is to be noted that Patent Reference 1 and Patent Reference 2 each disclose a technique for controlling only a maximum intermittent control frequency during intermittent oscillation.
The present invention, conceived to address the problems with the conventional techniques, has an object of providing a switching power supply apparatus and a semiconductor device each of which is controlled so as to avoid intermittent oscillation at a specific frequency band within an audible frequency range without using a control method such as a PWM control, a PFM control, or a quasi-resonant control, for the purpose of effectively reducing noises which components such as a transformer and a ceramic capacitor used in a switching power supply that performs intermittent-oscillation control under a light load, generates by operating at an audible frequency band.